A restricted blood supply to the heart muscle is a condition termed myocardial ischemia which is evidenced by abnormal heart muscle electrical activity. Untreated, myocardial ischemia can ultimately result in heart failure. As a result, monitoring of the electrical signals corresponding to heart muscle activity is an invaluable diagnostic tool for determining the health of the heart and identifying abnormalities thereof.
When the electrical signal of a heart muscle is plotted over time, it defines a characteristic curve having a waveform which extends periodically above and below a horizontal reference axis conventionally termed the isoelectric line. Each elevation or depression of the signal curve above or below the reference axis respectively is termed a wave and is identified by a letter. There are a total of six waves in each period of the signal which are identified by the letters, P, Q, R, S, T, and U. A straight line connecting two waves of the signal curve is further identified as a segment, while a wave and connecting straight line is termed an interval. Segments and intervals are identified by various combinations of the above-listed letters.
A normal electrical signal of a healthy heart muscle is generally reflected in a regular curve having predictable PR and ST segments and PR, QRS, and QT intervals. Anomalous electrical signals of a heart muscle are reflected by deviations in specific portions of the curve from the predicted norm. Such deviations may be symptomatic of myocardial ischemia.
More particularly, the ST segment deviations are known to be a key indicator of myocardial ischemia. The ST segment of a typical healthy heart is a straight line of zero slope on or near a horizontal reference axis. If the ST segment is parallel to the reference axis, but is elevated or depressed by a significant deviation from the reference axis, the heart muscle signal is termed anomalous which may be indicative of an unhealthy heart muscle. Likewise, an ST segment exhibiting a significant positive or negative slope, may further be indicative of an unhealthy heart.
Conventional electrical monitoring devices exist which are capable of characterizing the ST segment. However, such devices are usually relatively immobile and complex to operate which requires them to be maintained in a central medical facility for operation by skilled personnel. As a result, outpatients at such facilities only receive monitoring periodically and for relatively short time durations. Diagnosis of myocardial ischemia generally requires the compilation of signal histories for an extended period of time which periodic monitoring does not provide.
Portable monitors for heart muscle electrical activity represent a potential solution to this problem. Unfortunately, however, satisfactory portable monitors have not been developed which are compact, yet which have the capabilities of a clinical monitor. Known portable devices are often not sufficiently accurate or sensitive to detect small deviations in the heart muscle electrical signal, and particularly in the ST segment. Such deviations can be critical to the diagnosis of myocardial ischemia.
One reason for the lack of accuracy and sensitivity in portable monitors is that high-quality diagnostic data acquisition requires a relatively large power source. However, portable monitors by necessity rely on relatively small disposable or rechargeable power packs. The frequency with which the user would have to replace or recharge the power packs to produce diagnostic data equivalent to that produced by clinical monitors would be so great with known technology as to render such monitors impracticable.
As such, a solution to these problems is needed. Specifically, a low energy consumptive portable heart monitor is needed for acquiring diagnostic data. A portable heart monitor is particularly needed for acquiring data relating to the ST depression which is sufficiently sensitive to enable adequate detection and quantification of myocardial ischemia.